This research is directed toward understanding 1) the requirements for successful implantation and growth of the malignant canine mast cell tumor in homologous and heterologous species and 2) the host factors which modify tumor growth after implantation. The tumor is readily transplanted in beagle pups of less than four weeks of age, but its lack of growth in older animals implies that immunological factors are elaborated by immunologically-competent hosts. Whether this putative immunological control is derived from natural tumor-specific transplantation antigens of the mast cell or from canine histocompatibility antigens which have been incorporated into the tumor cell is unknown. The answer to this fundamental question is approached by cytotoxicity methods for cell-mediated and humoral immunity.